Methods are known for obtaining polyimide resins by polycondensing a tetracarboxylic acid dianhydride with an aromatic diamine in an organic solvent to prepare a polyamic acid as a precursor of a polyimide resin, followed by the dehydration and cyclization thereof by a chemical dehydration reaction under heating or using a dehydrating agent.
Generally, the ring opening polycondensation reaction proceeds in an organic solvent in such an amount as to give a polyamic acid concentration of 5 to 20% by weight in the solution. The solvent is removed from the solution of the polyamic acid uniformly dissolved in the organic solvent to obtain a film or a molded article.
Polyimide resins are widely used in the fields of electrical and electronical materials, adhesives, coatings, composite materials, fibers and film materials, because they are excellent in heat resistance, wear resistance, chemical resistance, electrical insulating properties and mechanical characteristics.
Polyimide resin films are also used in various ways as coating materials for electric wires, cables and wires and as insulating materials for transformers and printed-wiring boards, because of their excellent characteristics.
Generally, polyimide films are prepared by a method wherein an organic solution of a polyamic acid is cast on a support and the dehydration-cyclization reaction of the polyamic acid is accelerated by a heat treatment or a chemical treatment to obtain a polyimide film.
Alternatively, after an organic solvent solution of a polyamic acid is cast on a support, the solvent is removed by drying or the cast polyamic acid together with the support is immersed in a coagulating bath to coagulate the polymer solution, whereby a film can be obtained. The thus-obtained film is sometimes called a gel film. However, this gel film is formed by entanglement of polymer chains and does not have a three-dimensional network molecular structure formed through chemical bonds, therefore it is different from a gel film which is obtained by the present invention, which has a three-dimensional network structure.
JP-A-57-109614 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes a method wherein an organic solvent solution of a dehydrating agent and a polyamic acid and an organic solvent solution of a catalyst and a polyamic acid and are simultaneously applied to a support. After a gel film (obtained by entanglement of polymer chains) of the polyamic acid is formed by drying, the gel film is heat treated to obtain a polyimide film.
In the production of polyimide films, films having high mechanical characteristics can be obtained depending on the combinations of tetracarboxylic acid dianhydrides and aromatic diamines to be polycondensed. A polyimide film obtained from the combination of pyromellitic anhydride with 4,4'-diaminodiphenyl ether is a typical film which has high heat resistance and is excellent in tensile properties.
However, when heat resistance becomes higher, the film generally is brittle, and hence heat resistance is balanced against mechanical characteristics. Particularly, polyimide obtained from a combination of pyromellitic dianhydride with p-phenylenediamine has poor film properties and is brittle and extremely inferior in mechanical characteristics, though it is among the best with regard to physical and chemical heat resistance.
In the polyimide of this type, the mechanical characteristics of the polyimide film can be improved by copolymerization techniques. To improve mechanical characteristics without reducing heat resistance, JP-A-63-254131 discloses a method wherein 4,4'-diaminodiphenyl ether as another component of the aromatic diamine is copolymerized in the production of polyimide by using a combination of pyromellitic dianhydride with p-phenylenediamine. However, a large amount of 4,4'-diaminodiphenyl ether must be added to improve film forming properties and as a result, a loss of heat resistance results.
As used herein, the term "free-standing gel" means that the gel is capable of retaining its shape without causing deformation at room temperature (25.degree. C.). For example, when the gel is formed on a support as a film, the film is capable of retaining its shape without flowing even if the support is inclined.